Watermarks are often used to embed information into an image imperceptibly. However, many of the watermarks in general use are destroyed by distortions, such as scaling, rotation, shearing, and anamorphic scaling. Some watermark detection methods are even sensitive to translations.
Various methods have been proposed for detecting watermarks, with some of those methods said to be invariant to affine distortions. However, most methods that are invariant to affine distortions require, at some stage, an extensive search through a space of parameters defining the affine distortions. Such searches may cover many dimensions and typically consume considerable computing capacity.
Recently some methods have been proposed that reduce the search space by performing transformations which first remove the translation, then convert scaling and rotation into further translation effects in two orthogonal directions. These methods are known as RST (Rotation, Scale, and Translation) invariant methods. Typically such techniques require complementarity of the embedding and detection procedures.
Other techniques rely on embedding patterns with special symmetry properties, such as rotational symmetry, and then detecting those patterns by extensive search over the non-symmetric distortion parameter(s).
As none of the aforementioned methods imbue full affine distortion invariance, an extensive search through one or more of the distortion parameter spaces is still required to ensure full invariance. Accordingly, there is a need for a method of estimating the parameters of the affine distortion without requiring an extensive search through a parameter space.